1. Field of the invention
The present invention is generally concerned with clutches, in particular clutches for automotive vehicles.
2. Description of the prior art
As is well known, a clutch generally comprises a flywheel or reaction plate which is adapted to be constrained to rotate with a first shaft, generally a driving shaft, the motor crankshaft in the case of an automotive vehicle, a friction disk which is adapted to be constrained to rotate with a second shaft, generally a driven shaft, the input shaft of the gearbox in the case of an automotive vehicle, a clutch cover which is constrained to rotate with the flywheel, a pressure plate which is constrained to rotate with the cover and is movable axially relative to the cover, and axially acting spring means which, bearing against the cover, are adapted to urge the pressure plate towards the flywheel to clamp the friction disk against the latter and so engage the clutch.
The present invention is more particularly directed to the case where the flywheel is a damper type flywheel, by which is meant a flywheel within which are incorporated means for damping torsional torques.
As is also well known, in order to filter out vibrations that can arise at any point in the kinematic system that a transmission constitutes, extending from the motor to the axles in an automotive vehicle, it is usual to insert a torsional damper device into the transmission.
This torsional damper device is usually incorporated into the friction disk of the clutch.
It has already been proposed, however, as an alternative to this arrangement, or in conjunction with it, to incorporate a torsional damper device of this kind into the flywheel for certain specific applications, especially for vehicles in which the motor develops a relatively high torque at low rotation speeds, for which a specific requirement is to minimize the so-called "trash" noise which is generated when torque is applied.
A damper type flywheel of this kind generally comprises two parts rotatable relative to each other against circumferentially acting spring means, namely a first part through which it is adapted to be constrained to rotate with the first shaft, meaning the driving shaft in this instance, and which to this end comprises a flange constituting a support flange for the assembly and a second part against which the friction disk is adapted to bear axially and which to this end comprises a plate forming the reaction plate proper facing the friction disk.
In practise a damper type flywheel of this kind itself give rise to a problem because, being formed in two parts and thus comprising two inertial masses between which spring means are operative, it constitutes a resonant assembly in which resonance can occur as a result of the cylindrical loading to which it is subjected.
To alleviate this problem it is usual to incorporate into it a torque limiter with friction facings clamped up by axially acting spring means, this torque limiter being in practice disposed within the second of its constituent parts, by which is meant that comprising the reaction plate proper.
If, as a result of resonance, the damper type flywheel is subjected to a torsional torque that is greater than a predetermined value, chosen so as to be greater in all circumstances than the maximum motor torque that has to be transmitted, slipping occurs inside the relevant part of the damper type flywheel so that parts and equipment on the downstream side are not affected.
In practise the torque limiter with which the damper type flywheel is fitted in this way is operative only on starting and stopping the motor and on relatively rare occasions in response to impulses in the transmission due, for example, to excessively sudden engaging of the clutch.
However, use of a torque limiter of this kind in itself constitutes a problem.
Among other things, it is necessary to adjust the loading, that is to say the axial clamping force exerted by the axially acting spring means that control it, so that, as already mentioned, the corresponding torsional torque is in all circumstances greater than the maximum motor torque to be transmitted.
A general objective of the present invention is an arrangement which makes it possible to circumvent this problem and confers other advantages, at least in the case where it is possible to start up the motor of an automotive vehicle without first disengaging the clutch, which in practise corresponds to a significant fraction of the market.